1. Field of the Invention
The invention refers to a heart stimulator for stimulating at least one chamber of a heart by means of electrical stimulation pulses. The invention particularly refers to implantable pacemakers and implantable cardioverter/defibrillators featuring automatic capture threshold search.
2. Description of the Related Art
Implantable heart stimulators can be used for treating a variety of heart disorders like bradycardia, tachycardia or fibrillation by way of electric stimulation pulses delivered to the heart tissue, the myocardium. In order to be effective, a stimulation pulse needs to have strong enough of a strength to excite the myocardium of the heart chamber to be paced. Excitation of the myocardium that in turn is followed by a contraction of the respective heart chamber. The stimulation pulse strength high enough to cause excitation of the myocardium is called capture threshold since strong enough a stimulation pulse causes capture.
Depending on the disorder to be treated, such heart stimulator generates electrical stimulation pulses that are delivered to the heart tissue (myocardium) of a respective heart chamber according to an adequate timing regime. Delivery of stimulation pulses to the myocardium is usually achieved by means of an electrode lead that is electrically connected to a stimulation pulse generator inside a heart stimulator's housing and that carries a stimulation electrode in the region of its distal end. A stimulation pulse also is called a pace. Similarly, pacing a heart chamber means stimulating a heart chamber by delivery of a stimulation pulse.
In order to be able to sense a contraction a heart chamber that naturally occurs without artificial stimulation and that is called intrinsic, the heart stimulator usually comprises at least one sensing stage that is connected to a sensing electrode on the electrode placed in the heart chamber. An intrinsic excitation of a heart chamber results in characteristic electrical potentials that can be picked up via the sensing electrode and that can be evaluated by the sensing stage in order to determine whether an intrinsic excitation (called an intrinsic event), has occurred.
Usually, a heart stimulator features separate stimulation generators for each heart chamber to be stimulated. Therefore, in a dual chamber pacemaker, usually an atrial and a ventricular stimulation pulse generator for generating atrial and ventricular stimulation pulses are provided. Delivery of an atrial or a ventricular stimulation pulse causing an artificial excitation of the atrium or the ventricle, respectively, is called an atrial stimulation event AP (atrial paced event) or a ventricular stimulation event VP (ventricular paced event), respectively. The strength of stimulation pulses delivered by the respective stimulation pulse generator is adjustable in order to be able to adjust the stimulation pulse strength to be just sufficient to cause capture (above capture threshold) and thus using as little energy as possible to be effective. Stimulation pulse strength depends on both, duration and amplitude of the stimulation pulse. Thus, stimulation pulse strength can be adjusted by varying pulse duration, pulse amplitude or both. Usually, pulse strength is adjusted by altering the pulse amplitude.
Common heart stimulators feature separate sensing stages for each heart chamber to be of interest. In a dual chamber pacemaker usually two separate sensing stages, an atrial sensing stage and a ventricular sensing stage, are provided that are capable to detect intrinsic atrial events AS (atrial sensed event) or intrinsic ventricular events VS (ventricular sensed event), respectively.
As known in the art, separate sensing and pacing stages are provided for three-chamber (right atrium RA, right ventricle RV, left ventricle LV) or four-chamber (right atrium RA, left atrium LA, right ventricle RV, left ventricle LV) pacemakers or ICDs.
By means of a sensing stage for a heart chamber to be stimulated, the pacemaker is able to only trigger stimulation pulses when needed that is when no intrinsic excitation of the heart chamber occurs in time. Such mode of pacing a heart chamber is called demand mode. In the demand mode the pacemaker schedules an atrial or a ventricular escape interval that causes triggering of an atrial or ventricular stimulation pulse when the escape interval times out. Otherwise, if an intrinsic atrial or ventricular event is detected prior to time out of the respective atrial or ventricular escape interval, triggering of the atrial or ventricular stimulation pulse is inhibited. Such intrinsic (natural, non-stimulated) excitation are manifested by the occurrence of recognizable electrical signals that accompany the depolarization or excitation of a cardiac muscle tissue (myocardium). The depolarization of the myocardium is usually immediately followed by a cardiac contraction. For the purpose of the present application, depolarization and contraction may be considered as simultaneous events and the terms “depolarization” and “contraction” are used herein as synonyms. The recognizable electrical signals that accompany the depolarization or excitation of a heart chamber are picked up (sensed) by the atrial or the ventricular sensing channel, respectively. Thus, by means of the sensing stages, intracardiac electrogram (IEGM) signals are acquired, that can be evaluated by the heart stimulator. Simple evaluation only checks whether the IEGM exceeds a given threshold in order to detect a sense event. More complex evaluation includes analysis of the IEGM's morphology.
In heart cycle, an excitation of the myocardium leads to depolarization of the myocardium that causes a contraction of the heart chamber. If the myocardium is fully depolarized it is unsusceptible for further excitation and thus refractory. Thereafter, the myocardium repolarizes and thus relaxes and the heart chamber is expanding again. In a typical electrogram (EGM) depolarization of the ventricle corresponds to a signal known as “R-wave”. The repolarization of the ventricular myocardium coincides with a signal known as “T-wave”. Atrial depolarization is manifested by a signal known as “P-wave”.
A natural contraction of a heart chamber thus can be detected by evaluating electrical signals sensed by the sensing channels. In the sensed electrical signal the depolarization of an atrium muscle tissue is manifested by occurrence of a P-wave. Similarly, the depolarization of ventricular muscle tissue is manifested by the occurrence of an R-wave. A P-wave or an R-wave thus leads to an atrial sense event As or a ventricular sense event Vs, respectively.
Picking up and evaluating electric signals in a heart chamber by a sensing stage also is used to control effectiveness of a stimulation pulse as pointed out in further detail below.
Automatic capture threshold search serves for determining an optimum stimulation pulse strength that is just sufficient to capture the heart tissue and that does not include an excessive safety margin. Furthermore, some devices include an automatic capture control feature, which checks whether or not a delivered pace pulse has resulted in capture of the heart tissue on a beat-to-beat basis and adjust the pace energy as warranted by the situation—with this feature active, the safety margin can be further reduced.
Several modes of operation are available in a state of the art multi mode pacemaker. The pacing modes of a pacemaker, both single and dual or more chamber pacemakers are classified by type according to a three letter code. In such code, the first letter identifies the chamber of the heart that is paced (i.e., that chamber where a stimulation pulse is delivered), with a “V” indicating the ventricle, an “A” indicating the atrium, and a “D” indicating both the atrium and ventricle. The second letter of the code identifies the chamber wherein cardiac activity is sensed, using the same letters, and wherein an “O” indicates no sensing occurs. The third letter of the code identifies the action or response that is taken by the pacemaker. In general, three types of action or responses are recognized: (1) an Inhibiting (“I”) response wherein a stimulation pulse is delivered to the designated chamber at the conclusion of the appropriate escape interval unless cardiac activity is sensed during the escape interval, in which case the stimulation pulse is inhibited; (2) a Trigger (“T”) response wherein a stimulation pulse to a prescribed chamber of the heart a prescribed period of time after a sensed event; or (3) a Dual (“D”) response wherein both the Inhibiting mode and Trigger mode may be evoked, e.g., with the “inhibiting” occurring in one chamber of the heart and the “triggering” in the other.
To such three letter code, a fourth letter “R” may be added to designate a rate-responsive pace-maker and/or whether the rate-responsive features of such a rate-responsive pacemaker are enabled (“O” typically being used to designate that rate-responsive operation has been disabled). A rate-responsive pacemaker is one wherein a specified parameter or combination of parameters, such as physical activity, the amount of oxygen in the blood, the temperature of the blood, etc., is sensed with an appropriate sensor and is used as a physiological indicator of what the pacing rate should be. When enabled, such rate-responsive pacemaker thus provides stimulation pulses that best meet the physiological demands of the patient.
A dual chamber pacemaker featuring an atrial and a ventricular sensing stage and an atrial and a ventricular stimulation pulse generator can be operated in a number of stimulation modes like VVI, wherein atrial sense events are ignored and no atrial stimulation pulses are generated, but only ventricular stimulation pulses are delivered in a demand mode, AAI, wherein ventricular sense events are ignored and no ventricular stimulation pulses are generated, but only atrial stimulation pulses are delivered in a demand mode, or DDD, wherein both, atrial and ventricular stimulation pulses are delivered in a demand mode. In such DDD mode of pacing, ventricular stimulation pulses can be generated in synchrony with sensed intrinsic atrial events and thus in synchrony with an intrinsic atrial rate, wherein a ventricular stimulation pulse is scheduled to follow an intrinsic atrial contraction after an appropriate atrioventricular delay (AV-delay; AVD), thereby maintaining the hemodynamic benefit of atrioventricular synchrony.
The energy needed for a stimulation pulse is delivered by a depletable battery that cannot easily be exchanged. Therefore the energy used for a stimulation pulse shall be a little as possible without affecting effectiveness of the stimulation pulse.
A stimulation pulse to the myocardium only effectively excites the myocardium and thus causes capture of a respective heart chamber, if the myocardium of that chamber is not in a refractory state and if the stimulation pulse strength is above the capture threshold of the myocardium. A sub-threshold stimulation pulse will not cause capture even if delivered to the myocardium in its non-refractory state. Capture only occurs if a stimulation pulse is strong enough to cause excitation of the myocardium. Pulse strength depends both on duration and amplitude of an electrical stimulation pulse. Usually, stimulation pulse strength is adjusted by adjusting the pulse amplitude while maintaining the pulse duration.
It is desirable to adjust the stimulation pulse strength so that the stimulation pulse strengths for particular heart chambers just above capture threshold in order to spend as little energy as possible for a single stimulation pulse while ensuring reliable effectiveness of a stimulation pulse delivered.
Since capture threshold may vary from heart chamber to heart chamber and from patient to patient and may even vary over time, there is a need for automatic capture testing and determination in particular as far as implantable heart stimulators are concerned.
In order to determine whether a stimulation pulse has let to capture or not, modern heart stimulators provide for an automatic capture threshold search and/or automatic capture control after delivery of a stimulation pulse.
During automatic capture threshold search stimulation pulses of different strength are tested—at least once daily—in order to determine individual capture threshold to which only a small safety margin needs to be added. With heart stimulators that include an automatic capture control feature, which checks whether or not a delivered pace pulse has resulted in capture of the heart tissue on a beat-to-beat basis it is possible to adjust the stimulation pulse strength as warranted by the situation. With this feature active, the safety margin can be further reduced.
Both of the features mentioned above, i.e. the automatic capture threshold search and the automatic capture control may be based on analyzing the post-pace artifact in the EGM signal. Basically, a capture event is identified when the post-pace artifact deviates from the known non-capture artifact by a significant amount for one or more characteristics of the waveform.
In some cases a stimulated excitation occurs simultaneously with an intrinsic excitation and causes a so-called fusion beat. It should be noted that a fusion beat can result in incorrect conclusions—the event may be classified as a capture or as a non-capture depending upon, and not limited to, the timing of the intrinsic beat and the pace delivery.
From the above it becomes clear that evaluation of the post-pace artifact to conclude whether or not the pace pulse has resulted in a capture may produce unreliable results due to fusion beats. This unreliability in capture classification (CC) has the potential of producing a few undesirable wrong conclusions. During an automatic capture threshold search (CTS) in the ventricle, an incorrect non-capture classification that may be due to a fusion beat can lead to a determination of the capture threshold as being higher than what it really is. During automatic capture control (ACC), incorrect non-capture classifications may lead to unnecessary backup pace pulses and/or unnecessary initiations of CTS.
The problem of fusion beats and they are to be avoided be adapting a programmable AV-delay is illustrated in EP 0 600 631.
While performing a capture threshold search in the ventricle with the bradycardia support mode programmed to be atrium-synchronous, e.g. DDD, fusion beats in the ventricle can be avoided by programming an AV-delay that is short enough to ensure that the ventricular stimulation pulse—having a pulse strength to be tested—is delivered well before any conducted intrinsic event can occur in the ventricle. In normal cases, this approach results in reliable capture classifications by analyzing the post-pace artifact in the EGM signal.
There is, however, a possibility that a premature atrial contraction (PAC) may result in a conducted ventricular depolarization right before or at the same time as the ventricular stimulation pulse having a pulse strength to be tested is delivered—and this fusion beat, as described earlier, may cause an incorrect capture classification. In case of an incorrect non-capture classification, this can ultimately lead to determination of the capture threshold being higher than what it really is; in some cases, the CTS may take too long time and may timeout resulting in a failed test.
For the purpose of this disclosure, the following abbreviations are used are used:
AbbreviationMeaningACCAutomatic Capture ControlApAtrial pace (stimulation) eventArsrefractory atrial sense eventAsAtrial sense eventAAny atrial eventAVDAV delay as applied by the pacemaker(in contrast to intrinsic AV delay)CCCapture ClassificationCRTcardiac resynchronization therapyCTSautomatic Capture Threshold SearchPACpremature atrial contractionPVARPPost-ventricular atrial refractory periodVAIVA interval (duration of the VA timer)VESVentricular extra-systoleVpVentricular pace (stimulation) eventVsVentricular sense eventVAny ventricular eventVTVentricular tachycardiaVFventricular fibrillation